Co-pending application Ser. No. 08/923,250 filed Sep. 4, 1997, now U.S. Pat. No. 5,904,809 discloses a foam-laid process, and apparatus for practicing the process, for forming nonwoven fibrous webs which increase the uniformity of the basis weight profile of the nonwoven web produced. The invention provides a manifold, and method, facilitating production of a nonwoven web by the foam process which is a modification of the method and apparatus in said co-pending application by which it is also possible to increase the uniformity of the basis weight profile, allowing a basis weight variation of less than 0.5%, and in fact as low as 0.2% and even lower, depending upon the fibers utilized.
The profile of the nonwoven web produced by the foam process is very much dependent upon the manifold distribution tube construction and design. In the liquid process which uses water, and nearly Newtonian liquids, one tries to make the profile uniform by adjusting both static and dynamic pressure characteristics of the fiber-liquid slurry, including by changing the shape of the back wall of the manifold, and by varying the pressure in the manifold by controlling an outlet valve from the manifold. Foam-fiber-surfactant slurry, however, behaves differently than Newtonian, or near Newtonian liquids, making adjustments of the profile difficult when utilizing conventional manifold pipes. These problems can be greatly magnified if the particular fibers (or particles in the slurry) do not properly flow through the outlet valve in the manifold, are unstable in water, are sensitive to flocculation, or to build-up of knots or filter bundles.
According to the present invention, a manifold facilitating production of a nonwoven web using the foam process, and the foam process for producing nonwoven webs using the manifold, are provided which allow precise control of manifold pressure locally, and simultaneously over substantially the entire length of, the manifold. The web profile and formation can be precisely controlled. Control can be effected by one, or preferably all of, the back pressure established by controlling the outlet valve, feed rate to the manifold, and the feed rate of substantially fiber-free foam into the back wall of the manifold.
According to one aspect of the present invention a manifold facilitating production of a nonwoven web of fibrous material is provided comprising the following components: A manifold casing comprising first and second opposite ends, including an inlet for a foam-fiber slurry at the first end. A center section of the manifold casing having a substantially decreasing effective cross-sectional area from the inlet to the outlet. First and second side walls, a front wall having an effective length, and a back wall, of the center section. The front wall being porous to the foam-fiber slurry to allow passage of the slurry therethrough. Means for introducing a second foam (e.g. substantially fiber free, or a foam fiber slurry, which may include surfactant) into the center section through the back wall. And, the means for introducing the second foam (and perhaps the shape and dimensions of the center section) being constructed so as to facilitate maintaining the basis weight of foam-fiber slurry passing through the front wall substantially constant along the effective length of the front wall.
A significant feature of the manifold is a decreasing cross sectional area from the inlet towards the outlet. The decrease of the cross-sectional area depends on three factors; the amount of slurry discharged from the manifold towards the headbox, the kinetic energy of the slurry inside the manifold, and the surface friction between the manifold walls and the slurry. The manifold may have any shape that takes these into account. For instance, the manifold could be a cylindrical pipe having a conical member therein for decreasing the cross-sectional area. In such a structure the nozzles leading the slurry out of the manifold may be positioned around the cylindrical manifold at all directions, and the pipes supplying the second foam could be disposed at the conical pipe inside the manifold. In this case the side walls and the front wall and back wall are part of a continuous curved structure. In fact, the cross-section of the entire front wall, back wall, and side walls are preferably cylindrical.
Alternatively, the manifold could be two sided i.e. the nozzles attached to the opposite sides of the manifold so that the fiber free foam could be introduced through the other opposite walls where the cross-section of the manifold is rectangular.
The orientation of the manifold typically has very little significance; it may be disposed either in an upright, inclined or horizontal position.
The means for introducing the substantially fiber-free foam into the center section through the back wall may comprise any conventional fluid components including nozzles, perforated plates, baffles, spray heads, or the like. Preferably such means comprises one or more lines of valved pipes, the valves being controllable to vary the amount of foam passing therethrough.
In the preferred embodiment of the manifold the back wall of the center section slopes with respect to the front wall so that the back wall becomes closer to the front wall, and the cross-sectional area of the center section becomes smaller, moving from adjacent the first end of the manifold toward the second end. Preferably the side walls are substantially closed and the back wall is substantially closed except for the means for introducing substantially fiber free foam; and the manifold may further comprise an outlet at the second end of the manifold, in which case the fiber-foam mixture can be recirculated. [A valve may preferably be disposed in the outlet to vary the amount of slurry passing through the outlet.] The front wall may be substantially horizontal, or it may have other orientations. The manifold is typically provided with nozzles and conduits leading the slurry to a headbox, in combination with a moving foramininous element (such as a wire) on which a nonwoven web is formed by slurry passing through the front wall into the nozzles and conduits, and then into the headbox; and in a downstream former foam and liquid are sucked out of the slurry to form the web on the foramininous element.
The manifold may further comprise a plurality of pressure sensors operatively connected to at least one of the substantially closed side walls for sensing the pressure within the center section thereat. Still further the manifold may comprise control means responsive to the pressure sensors for controlling at least one of (preferably all of) introduction of foam-fiber slurry, withdrawal of foam-fiber slurry, and introduction of substantially fiber free foam, into the center section to maintain the basis weight of foam-fiber slurry passing through the front wall substantially constant along the effective length of the front wall. The control means may comprise any conventional type of computer control, fuzzy controller, a multi-variable control unit, or the like that cooperates with valves, baffles, or other conventional fluidic elements to perform the desired function automatically.
The cross-section of the center section may be a parallelogram, or a wide variety of other types of polygons or other shapes (as described above), but preferably is substantially rectangular. The manifold center section typically comprises a polygonal base prism, such as a rectangular base prism.
According to another aspect of the present invention a manifold facilitating production of a nonwoven web of fibrous material is provided comprising the following components: A manifold casing comprising first and second opposite ends, including an inlet for a foam-fiber slurry at the first end, outlet at the second end of the manifold, and a valve disposed in the outlet to vary the amount of slurry passing through the outlet. A center section of the manifold casing having a substantially polygonal cross-section. First and second side walls, a front wall having an effective length, and a back wall, of the center section. The front wall being porous to the foam-fiber slurry to allow passage of the slurry therethrough. Means for introducing a second foam into the center section through the back wall. And, wherein the back wall of the center section slopes with respect to the front wall so that the back wall becomes closer to the front wall, and the cross-sectional area of the center section becomes smaller, moving from adjacent the first end of the manifold toward the second end. The details of the manifold are preferably as described above.
The invention also relates to a method of producing a nonwoven web of fibrous material using a manifold having a front porous wall having an effective length through which foam-fiber slurry can flow, first and second ends separated along the effective length, and a back wall opposing the front wall; and a headbox. The method preferably comprises: (a) Substantially continuously introducing foam-fiber-surfactant slurry into the first end of the manifold. (b) Substantially continuously discharging foam-fiber-surfactant slurry through openings in the manifold front wall to be delivered to the headbox. And, (c) Introducing a second foam (e.g. substantially fiber free, or a fiber-foam slurry having approximately the same, or a different (e.g. by at least about 1%), percentage of fibers as the foam-fiber slurry introduced at (a)) into the manifold through a number of openings spaced at substantially regular intervals substantially over the entire length thereof, so as to maintain the basis weight of foam-fiber-surfactant slurry passing through the manifold front wall substantially constant along the effective length of the manifold front wall.
The method preferably further comprises (d) sensing the pressure in the manifold at a plurality of positions along the length thereof, and practicing (c) in response to the sensed pressure to maintain the basis weight of foam-fiber slurry passing through the front wall has a variation of less than 0.5% along the effective length of the front wall. Preferably (c) is also practiced substantially continuously. Preferably the manifold has a center section between the first and second ends thereof with a substantially polygonal cross-section that gradually decreases substantially along the effective length of the front wall, and in that case (c) is practiced so that the foam-fiber-surfactant slurry moves through the constantly decreasing cross-section of the center section. Also, the method typically further comprises (e) substantially continuously withdrawing some slurry through the second end of the manifold.
It is the primary object of the present invention to provide a manifold, and method of producing a nonwoven web of fibrous material utilizing the manifold, which takes into account the non-Newtonian aspects of the foam-fiber-surfactant slurries, to produce a nonwoven web of substantially constant basis weight along the effective length of the front wall of the manifold. This and other objects of the invention will become clear from an inspection of the detailed description of the invention, and from the appended claims.